std::tmpfile() not picking TMPDIR location

问题

I am using std::tmpfile() to create temporary files, but I want to use the location other than /tmp. I am exporting $TMPDIR to point to the new location, but std::tmpfile() doesn't pick the new location.

How to create temporary files with std::tmpfile() in a folder other than /tmp?

回答1:

A quick test program

#include <stdio.h>
#include <stdlib.h>
#include <cstdlib>
#include <iostream>

int main()
{
    const char* tf = std::tmpnam(nullptr);
    std::cout << "tmpfile: " << tf << '\n';
    return 0;
}

and an ltrace

osmith@osmith-VirtualBox:~$ ltrace ./test.exe
__libc_start_main(0x400836, 1, 0x7ffedf17e178, 0x4008e0 <unfinished ...>
_ZNSt8ios_base4InitC1Ev(0x601171, 0xffff, 0x7ffedf17e188, 160)                  = 0
__cxa_atexit(0x400700, 0x601171, 0x601058, 0x7ffedf17df50)                      = 0
tmpnam(0, 0x7ffedf17e178, 0x7ffedf17e188, 192)                                  = 0x7fe4db5a0700
_ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc(0x601060, 0x400965, -136, 0x7fe4db2d13d5) = 0x601060
_ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc(0x601060, 0x7fe4db5a0700, 0x601060, 0xfbad2a84) = 0x601060
_ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_c(0x601060, 10, 0x7fe4db91d988, 0x57474f44696b656ctmpfile: /tmp/filekiDOGW
) = 0x601060
_ZNSt8ios_base4InitD1Ev(0x601171, 0, 0x400700, 0x7fe4db59fd10)                  = 0x7fe4db922880
+++ exited (status 0) +++

confirms that, per the man page, no environment variables are consulted by std::tmpnam, it only uses P_tmpdir which is a constant.

If this is purely for Linux, you could use mkstemp instead:

#include <stdio.h>
#include <stdlib.h>
#include <cstdlib>
#include <iostream>
#include <unistd.h>

int main()
{
    char tmpl[] = "/var/tmp/testXXXXXX";
    int f = mkstemp(tmpl);
    if (f < 0) {
        std::cerr << "mkstemp failed\n";
        return 1;
    }
    std::cout << tmpl << '\n';
    close(f);
    return 0;
}

Demo:

osmith@osmith-VirtualBox:~$ g++ -o test.exe test.cpp -std=c++14
osmith@osmith-VirtualBox:~$ ./test.exe
/var/tmp/testEFULD4

来源：https://stackoverflow.com/questions/38284450/stdtmpfile-not-picking-tmpdir-location